1. Field of Invention
This invention uses multiple observers to passively determine range and bearing to an RF emitter. In particular, it employs ambiguous emitter wavefront phase change measured at each of at least two moving aircraft, and pulse time of arrival measurements made between two platforms to perform the geolocation.
2. Description of Related Art
Applicant's copending application entitled, "COMBINED PHASE-CIRCLE AND MULTIPLATFORM TDOA PRECISION EMITTER LOCATION," filed on even date herewith and assigned to present assignee, the entire disclosure of which is hereby incorporated by reference in this specification, discloses a method for reducing the geometrical dilution of precision (GDOP) degradation experienced when using multiplatform circular lines-of-position for precision emitter location. This was accomplished by combining the circles-of-position (COPs) generated by interferometer phase difference measurements made by one moving platform with hyperbolic lines-of-position (HLOPs) generated by pulse time difference of arrival (TDOA) measurements between two observers. This combined technique overcame low-frequency limitations in the phase-circle-only approach and reduced sensitivity to time of arrival (TOA) measurement errors compared with TDOA-only geolocation. The use of measurements made by a fully resolved short baseline interferometer (SBI) on the moving platform was a key element of the method. For example, the SBI angle-of-arrival (AOA) measurements made by two separate platforms were used to provide an initial coarse location of the emitter. This coarse location was accurate enough to verify that TDOA measurements between observers were being made on the same pulse of the emitter signal. The SBI was also used in more fundamental ways to generate the phase circles, or COPs, as discussed briefly below.
The COPs were produced from long-baseline interferometer differential phase measurements by the method described in the applicant's U.S. Pat. No. 5,526,001. Resolving ambiguous LBI phase requires simultaneously measuring fully resolved phase with the SBI, using the technique disclosed by Kaplan in U.S. Pat. No. 4,734,702.
In this approach for generating phase circles, requirements on the system phase measurement's repeatable accuracy were reduced. This reduction occurred because any fixed-phase bias error present during the receiver dwell at the first observation point canceled when forming the phase difference at the second observation point. Thus, the LBI baseline did not require calibration. Also, constant antenna phase mistrack errors and receiver calibration phase-bias errors canceled and had no impact on COP accuracy. But as a consequence of this method, the LBI measured only angle change, and not AOA. The SBI not only predicted angle change to resolve the LBI, but also provided the measure of AOA required to correct for variable bias errors.
Variable bias errors, i.e., those that remain constant during a receiver dwell but vary from one receiver dwell to another, greatly impact COP precision and, hence, location accuracy, and must be reduced. The most significant variable bias error is due to changes in a scanning radar's electromagnetic wave polarization caused by the observer detecting different emitter sidelobes in different receiver dwells. This dwell-to-dwell polarization change affects the LBI antenna phase mistrack, and can cause in the differential phase measurement an error of five electrical degrees or more. Since the error is constant between all signal pulses used to form the phase difference between dwells, it cannot be reduced by averaging, as can thermal noise and quantization errors. The method to reduce this error disclosed in applicant's copending application, entitled, "COMBINED PHASE-CIRCLE AND MULTIPLATFORM TDOA PRECISION EMITTER LOCATION," contemplates producing a table from antenna polarization response measurements on the observing aircraft for different signal angles-of-arrival. The SBI AOA measurements made when LBI phase differences are formed are then used to access this table for calibration data that corrects the LBI measurements.
Hence, for resolving the long-baseline interferometer, correcting the variable phase bias errors, and confirming the TDOA measurement, the SBI forms an intrinsic part of the approach given in applicant's copending application entitled, "COMBINED PHASE-CIRCLE AND MULTIPLATFORM TDOA PRECISION EMITTER LOCATION." However, many aircraft used to passively locate emitters do not currently have an SBI available. Furthermore, because of weight, cost, and airframe limitations, it may not be feasible to add an SBI to the existing electronic surveillance measurement (ESM) system. Therefore, it is desirable to have an alternative approach to implementing a combined COP-HLOP location technique that preserves the method's GDOP reduction, improved low-frequency performance, and reduced need for TDOA accuracy, while requiring only two antenna elements. This requires that alternatives be found to resolve the phase ambiguity and correct the phase polarization error.
One alternative approach to differentially resolving the LBI is disclosed in applicant's U.S. Pat. No. 5,343,212, and discussed in connection with phase-circle generation in U.S. Pat. No. 5,526,001. A set of emitter positions is postulated, and each used to establish a hypothesis test. The hypothesis test generates a set of potential emitter locations, resolves the LBI in a manner consistent with each of these assumed locations, and utilizes a sequential check over a number of measurements to determine the actual emitter location from the set. While robust, this method requires multiple receiver dwells to eliminate the incorrect emitter locations. In many multiplatform geolocation situations that are tactically important, the emitter may transmit for no more than ten seconds and the number of phase difference measurements made in that interval can be severely limited. Such a small number of measurements may not be sufficient for the hypothesis test to generate a single unambiguous phase circle.
This invention overcomes the limitations of using an SBI or hypothesis test to resolve the LBI at the expense of requiring at least two moving observers to separately generate multiple phase circles from the ambiguous LBI differential phase measurements. A phase circle is produced for each possible ambiguity resolution of the differential measurements. This is illustrated in FIG. 3 for the scenario shown in FIG. 2 208. The COPs 300, 301 and 302 are derived from the ambiguous phase measurements made by observer 308, corresponding to aircraft 206 in FIG. 2 208, while COP 304, 305 and 306 are generated from the ambiguous phase measurements made by aircraft 307 (205 in FIG. 2). COP 301 and 305 are the true emitter circles-of-position, and this is determined by the common intersection 309 with the TDOA hyperbola limb 303. In this example the TDOA is measured between platforms 307 and 308, but other observers could be used. Thus the invention does not attempt to correctly resolve the phase difference measurement ambiguities before generating the phase circles, and hence does not have the problem with sparse data that can degrade the hypothesis test method. In fact, the phase difference ambiguity is not resolved before the emitter is located. This creates difficulties in correcting the LBI phase measurements for variable bias error and overcoming this difficulty is a key aspect of the invention.
The use of the TDOA measurement in conjunction with the ambiguous phase measurements to locate the emitter and then the use of emitter location to resolve the differential phase should be compared with methods that use TDOA to directly resolve the LBI. Cusdin et al. in U.S. Pat. No. 4,797,679 provide an approach representative of such direct techniques. The LBI used in Cusdin's method must be phase calibrated, and the TDOA measurement is made on the same platform between the two antennas used to measure LBI phase. Also the TDOA measurement must be nearly simultaneous with the phase measurement. Cusdin's is thus intrinsically a single platform technique that associates resolved LBI phase with emitter signal AOA. For multiplatform geolocation the resolved AOAs on two platforms could be intersected, as shown in FIG. 1a. In this figure 160 and 161 represent the AOA, while 178 and 179 are the wedge shaped AOA errors, and 162 the uncertainty these errors create in the emitter location. This region of uncertainty grows quickly with range, but an advantage the method does have is that the observers 163 and 165 obtain the range estimate in a single observer dwell.
By contrast to the direct ambiguity resolution of calibrated LBI phase measurements by TDOA in a single receiver dwell, the method disclosed here uses uncalibrated LBI baselines that are differentially resolved across receiver dwells. Hence, as noted above, AOA is not measured and at least two separate receiver dwells made seconds apart, indicated by the moving observer at 165-166 and at 167-168, are required. Also the LBI ambiguities for the phase measurements made on at least two aircraft must be simultaneously resolved, in effect, by locating the emitter utilizing TOA measurements made on separate platforms rather than across the LBI baseline. The TOA measurement does not have to be time coincident with the LBI phase measurements, nor, as emphasized above, do the observers making the TOA measurements have to be the same as those making the differential phase measurements. Thus this invention is intrinsically a multiplatform technique.
The association of the LBI differential phase measurements with COP 169 and 170 in 174 FIG. 1a rather than the LBI phase with AOA 160 and 161 (173 FIG. 1a) provides a substantial reduction in GDOP compared with a multiplatform application of Cusdin's method. Although the magnitude of the COP error region does have a range dependence since it is ultimately based on the bearing subtended at the emitter, the excursions caused by one sigma error variations indicated by 175 and 177 are the same at each point on the respective COPs 169 and 170. Further, since the TDOA measurement is made between platforms rather than between two antenna on a single platform, it provides a third LOP 171 (with range independent one sigma error 175) which greatly reduces the GDOP, as indicated by the emitter location error region 172.
FIG. 1b is a top level block diagram of the invention, illustrating how ambiguous LBI and TDOA measurements are used to geolocate the emitter in the manner indicated by 174 FIG. 1a for the FIG. 2 208 scenario, producing the COP and HLOP as shown in FIG. 3. Aircraft 307 in that figure corresponds to observer 100 FIG. 1a. The system on aircraft 308 corresponds to 102 and is identical to 100. Hence only the utilization of system 100 will be described in detail in the following summary. The central computing site 103 could be located on either aircraft, or on both, or on a third platform.
The operation of these particular features and other aspects of the invention, such as the method for reducing the impact of variable bias errors without making SBI measurements, are presented in more detail in the summary that follows.